Thermoelectric conversion materials, and thermoelectric conversion modules using thermoelectric conversion materials have been used as devices for converting heat into electricity and vice versa for cooling and electricity generation. For example, passing a direct current through a thermoelectric conversion material brings about transfer of heat from one surface to the other, and creates an endothermic surface and an exothermic surface. This phenomenon is known as the Peltier effect. By bringing the endothermic surface of a module prepared from a thermoelectric conversion material into contact with an object to be cooled, the Peltier effect allows cooling the object without requiring moving parts. On the other hand, applying a temperature difference across a thermoelectric conversion material creates a proportional electromotive force. This phenomenon is known as the Seebeck effect, which allows conversion of heat energy into electrical energy when one side of a module is cooled by air cooling or water cooling while the other side is in contact with an object dissipating excess heat energy. Specifically, the Seebeck effect allows for recovery of waste energy. Such thermoelectric conversion modules using the Seebeck effect have attracted interest as an electricity generating device, and use of thermoelectric conversion modules in such novel applications has been actively studied.
Bismuth-tellurium-based materials are the most well known example of materials that effectively produce the thermoelectric conversion phenomenon. Modules using bismuth-tellurium-based materials have been put to practical applications in cooling applications based on the Peltier effect, and in other applications, including temperature modulation of a laser diode for optical communications. There have been studies to use bismuth-tellurium-based-materials also for electricity generation. However, use of bismuth-tellurium-based materials in such applications is limited because of the temperature dependence of the electricity generating efficiency of thermoelectric conversion materials (bismuth-tellurium-based materials).
This is described below in more detail. One of the physical properties representing the characteristics of a thermoelectric conversion material is the Seebeck coefficient S (unit: V/K). This value, measured in volts per unit temperature difference, represents the magnitude of the electromotive force due to a temperature difference. The Seebeck coefficient takes a positive or a negative value, depending on the thermoelectric conversion material. This is determined by whether the carriers in the thermoelectric conversion material are holes or electrons. The notation, P-type or N-type, is typically used for positive and negative Seebeck coefficients, respectively. Electrical resistivity ρ (unit: Ω·m) is another physical property representing the characteristics of a thermoelectric conversion material. The generated electromotive force by the Seebeck effect produces a current that flows through a thermoelectric conversion material. However, the power that can be extracted for electricity generation is proportional to the product of the generated voltage and current. In other words, more power can be extracted when the electrical resistivity is low. Specifically, the foregoing two physical properties directly determine the electricity generating capability of a thermoelectric conversion material, and these are represented by a power factor PF (unit: W/mK2; hereinafter, also referred to simply as “PF”) calculated according to the following formula (1).
                    PF        =                              S            2                    ρ                                    (        1        )            
Thermal conductivity κ (unit: W/m·K) also represents the characteristics of a thermoelectric conversion material, though this is not a physical property that directly affects generation of electricity. In producing the Seebeck effect with a given amount of heat energy, a temperature difference does not easily occur in the material when the thermal conductivity of the thermoelectric conversion material is excessively large. Materials with scalier thermal conductivities thus allow for larger temperature, differences, and, in turn, larger amounts of electricity. The dimensionless performance index ZT combines the Seebeck coefficient S, the electrical resistivity ρ, and the thermal conductivity κ, as represented by the following formula (2).
                    ZT        =                                            S              2                                      ρ              ·              κ                                ×          T                                    (        2        )            
The dimensionless performance index ZT involves absolute temperature T (K) because the variables in the equation have temperature dependence. ZT is used as an index of thermoelectric conversion performance, not the amount of generated electricity itself, which is represented by PF, as described above. In other words, while ZT may take large values when the thermal conductivity is excessively small, the amount of generated electricity does not increase unless it is simultaneously accompanied by a large PF value.
FIG. 8 represents the PF of a bismuth-tellurium-based material as a function of temperature. As shown in FIG. 8, the highest PF value occurs in the vicinity of ordinary temperature in a bismuth-tellurium-based material, and the PF value has a tendency to decrease with increase in temperature.
A large temperature difference is needed to obtain large electricity with a thermoelectric conversion material. There are attempts for effective use of electricity converted from discharge heat of about 300° C. from engines and turbines of, for example, factories and automobiles. However, attempts to generate more electricity by making a large temperature difference, in a bismuth-tellurium-based material actually result in producing small PF values with increase in temperature, as shown in FIG. 8. Indeed, such temperature dependence has made it difficult to increase an amount of generated electricity, and there is a need for studies of a novel material.
JP-A-2007-5544 related to a thermoelectric-conversion material describes a cobalt-antimony-based material having high performance in a high temperature region as an alternative material of bismuth-tellurium-based materials. The cobalt-antimony-based material has a crystalline structure called a skutterudite structure. The composition formula is Co4Sb12, and the material has relatively large spaces inside the crystal lattice. Co4Sb12 itself is an N-type material, and has a desirable Seebeck coefficient. However, this material has a high electrical resistivity of, for example, about 1×10−4 Ω·m at ordinary temperature, and a high thermal conductivity of about 10 W/mK at ordinary temperature. Accordingly, PF and ZT are both low. It is known that adding other elements to Co4Sb12 improves the thermoelectric conversion characteristics, as described in JP-A-2007-5544. For example, both the electrical resistivity and the thermal conductivity can be reduced by adding Yb (ytterbium). It is also known chat thermal conductivity becomes effectively smaller in the presence of other elements, and this effect known as the rattling effect. The rattling effect occurs as the added Yb enters the spaces in the Co4Sb12 skeleton, which causes thermal oscillation independently from Co4Sb12, and reduces a phonon (lattice oscillation) in the Co4Sb12 skeleton.
However, the thermoelectric conversion material of the composition described in JP-A-2007-5544 is not sufficient for practical applications, and requires further improvement of thermoelectric characteristics.